The present invention relates to an A.F.C. (automatic frequency control) system for an FM radio receiver, and in particular to an A.F.C. system for a receiver to be utilized for reception of broad-band FM signals transmitted from an earth satellite.
In an FM receiver which utilizes a frequency synthesizer for tuning operation, the accuracy with which the frequency of the local oscillator is maintained will be identical to that of the reference frequency signal of the frequency synthesizer circuit. Usually a quartz crystal oscillator is utilized to produce the reference frequency signal, so that if the frequency accuracy of the recived signal is sufficiently high, a satisfactory degree of frequency accuracy for the I.F. (intermediate frequency) signal will be attained without the need to incorporate an A.F.C. system in the receiver. However in the case of a receiver system for reception of transmissions from an orbiting satellite, the received microwave band signal is first converted into a first I.F. signal by a down-converter (positioned close to the antenna, i.e. outdoors), and this signal is led to the receiver, situated indoors, through a cable. The first I.F. signal is then converted to a second I.F. signal within the receiver. Tuning, i.e. selection of a desired reception channel, is performed by the process of frequency conversion into the second I.F. signal, through variation of the frequency of the local oscillator signal which is employed in this conversion. With such a system, even if the local oscillator signal of the indoors receiver is generated by a frequency synthesizer circuit and hence has excellent frequency accuracy, satisfactory frequency accuracy will not be attained for the second I.F. if the conversion frequency accuracy of the down-converter, situated outdoors, is not sufficiently high. In practice it is found that a frequency drift of several MHz will occur due to ambient temperature variations, in the oscillation frequency of a local oscillator of a down-converter which is situated outdoors. In the case of the second I.F. signal however, even if the frequency accuracy is relatively poor, the amount of frequency drive will be held to within several hundred kHz. Thus, even if a frequency synthesizer circuit system is employed in the tuner circuit of such a receiver, it is essential to utilize an A.F.C. system in order to maintain the frequency accuracy of the second I.F. signal at a sufficiently high level. In the following, the term "local oscillator" will be restricted to signifying the local oscillator whose signal is utilized in producing the second I.F. signal, while the second I.F. signal will be simply referred to as the I.F. signal. An example of a prior art frequency synthesizer circuit type of tuner circuit, which is provided with an A.F.C. system, is described in Japanese Patent No. pb 55-23674. FIG. 1 is a block diagram of this prior art example, in which reference numeral 1 denotes a received signal input terminal, numeral 2 denotes a H.F. amplifier, numeral 3 denotes a frequency mixer, numeral 4 denotes a voltage-control type of local oscillator, numeral 5 denotes a PLL (phase lock loop) type of frequency synthesizer circuit, numeral 6 denotes an I.F. amplifier, numeral 7 denotes an FM demodulator, numeral 8 denotes a demodulated signal output terminal, numeral 9 denotes a low-pass filter (hereinafter referred to as LPF), and numeral 10 denotes a frequency error detection circuit. FIG. 2 shows an example of a circuit for the frequency error detection circuit 10, in which the demodulated output signal from the FM demodulator 7 is smoothed by transfer through a LPF 9, to thereby derive the DC component of the demodulated signal. This DC component is compared with fixed reference voltage levels V.sub.r1 and V.sub.r2 in a pair of voltage comparators 10a and 10b respectively. The DC component of the demodulated FM signal which is output from LPF 9 represents the average voltage level of the demodulated FM signal. This average voltage level corresponds to the average frequency of the I.F. signal which is produced by mixer 3. By comparing this average voltage level with the predetermined reference voltage levels V.sub.r1 and V.sub.r2 by the circuit shown in FIG. 2, it can be determined whether the center frequency of the I.F. signal has drifted from a predetermined I.F. frequency by more than a predetermined frequency range. In addition, the circuit detects the direction of this frequency outside the predetermined range, i.e. the output signals from terminals 10e, 10f respectively indicate whether the center frequency of the I.F. signal frequency is lower than or higher than the specified I.F. frequency. The output signals from terminals 10e and 10f are applied to the PLL frequency synthesizer circuit 5, which responds by performing fine adjustment of the frequency of oscillation of the local oscillator 4 such as to counteract the frequency drift, i.e. to maintain the amount of frequency drift outside the predetermined range to a sufficiently small amount.
However with the arrangement of FIG. 2, the frequency reference for A.F.C. system operation is constituted by the demodulator circuit 7 itself. In the case of a receiver system for reception of transmissions from an earth satellite, the FM demodulator does not display very good temperature stability, with respect to the relationship between input frequency and output voltage, since the most important design requirement for such a demodulator is that it must be capable of handling high frequency wide-band FM signals. For this reason, it is difficult to realize an A.F.C. system of the form shown in FIG. 1 which will maintain a high degree of I.F. signal frequency accuracy. If the FM signal must maintain a high degree of linearity in the demodulation process, as is true in the case of a video signal, then it is extremely important to ensure that amplitude and phase errors do not arise. If I.F. frequency drift occurs, then such errors will be produced as a result of passing the FM I.F. signal through the I.F. band-pass filter.
In order to overcome the problem described above, the assignee of the present invention has previously proposed (in Japanese patent application No. 60-205762, filed on Sept. 18, 1985) an A.F.C. system having the objective of overcoming the problems described above. In that A.F.C. system, a highly accurate reference frequency signal is utilized as a frequency reference. The FM I.F. signal is applied to the inputs of two frequency dividers having respectively different frequency division ratios. These frequency division ratios and the frequency of the reference frequency signal are selected such that, when the I.F. signal center frequency coincides with an upper limit frequency (which is higher than a predetermined I.F. frequency by a specific amount), the output of one of the frequency dividers will coincided in frequency with the reference signal, and such that when the I.F. signal center frequency coincides with a lower limit frequency (which is lower than the predetermined I.F. frequency by a specific amount), the output of the other frequency divider will coincide in frequency with the reference signal. Two frequency comparators are also utilized, with the reference frequency signal being applied to one input of each of these frequency comparators and the outputs from the frequency dividers being respectively applied to the other inputs of the frequency comparators. The resultant outputs signals from the frequency comparators are passed through respective integrator circuits or low-pass filters, to thereby produce two frequency comparison signals, whose levels indicate the frequency relationship between the reference frequency signal and the I.F. signal. These signals are applied to a frequency synthesizer circuit which produces a control voltage to control the local oscillator frequency, to thereby implement fine adjustment of that control voltage and hence fine control of the local oscillator frequency and hence the I.F. frequency, in accordance with the frequency relationship between the reference frequency signal and the I.F. signal.
With such an A.F.C. system, designating the upper and lower limits of frequency error detection with respect to the specified I.F. frequency as f.sub.H and f.sub.L, the reference signal frequency as f.sub.O, and the frequency division ratios of the frequency dividers as N.sub.H and N.sub.L respectively, then the following relationships must be satisfied: EQU f.sub.H =N.sub.H .multidot.F.sub.S ( 1) EQU f.sub.L =N.sub.L .multidot.F.sub.S ( 2) EQU f.sub.L &lt;f.sub.O &lt;f.sub.H ( 3)
It can thus be understood that when the center frequency of the I.F. signal is higher than the upper detection limit f.sub.H, the output frequency from the frequency divider having the frequency division ratio N.sub.H will become higher f.sub.H /N.sub.H, and so (from equation (1)) will become higher than the reference signal frequency F.sub.S. As a result, the output from the frequency comparator which receives the latter frequency divider output and the reference frequency signal will indicate that the center frequency of the I.F. signal has become higher than the upper detection limit f.sub.H. Similarly, when the center frequency of the I.F. signal is higher than the lower detection limit f.sub.L, the output frequency from the frequency divider having the division ratio N.sub.L will become lower than f.sub.L /N.sub.L, and so (from equation (2) above) will become lower than the reference signal frequency F.sub.S. As a result, the output from the frequency comparator which receives the latter frequency divider output and the reference frequency signal will indicate that the center frequency of the I.F. signal has become lower than the lower detection limit f.sub.L.
It is possible to utilize a digital circuit as such a frequency comparator, to perform phase (frequency) comparison. However a receiver for reception of earth satellite transmissions produces a wide-band IF signal, having a high value of FM modulation index. Thus, it is necessary to make the frequency division ratios N.sub.L and N.sub.H sufficiently high, in order to ensure that the FM modulation index of the output signals from these frequency division ratios will be low enough to ensure that erroneous operation of the frequency comparators does not occur as a result of the frequency deviation due to wide-band FM modulation of the I.F. signal.
The output signals from the frequency comparators are then passed through simple integrator circuits (or low-pass filters) to derive the DC component of each signal, to thereby obtain two frequency comparison signals. One of these frequency comparison signal will change between a high and a low logic level when the amount of error of the center frequency of the I.F. signal reaches the upper detection limit f.sub.H, while the other frequency comparison signal will similarly change in logic level when the I.F. signal reaches the lower detection limit f.sub.L. The combination of these two frequency comparison signal can be utilized to judge which of three possible conditions is currently true of the center frequency f.sub.C of the I.F. signal, i.e. f.sub.C &lt;f.sub.L, f.sub.L &lt;f.sub.C &lt;f.sub.H, or f.sub.H &lt;f.sub.C. Thus the frequency comparison signal can be utilized to judge whether fine adjustment of the local oscillator frequency should be performed (i.e. by fine adjustment of the control voltage applied to the local oscillator in the case of a receiver employing a synthesizer circuit to produce such a control voltage), and also the direction in which this fine adjustment of the local oscillator frequency is to be executed.
An example of such an A.F.C. system is shown in the block diagram of FIG. 3. As in the previous example, a received signal (i.e. from a down-converter) is transferred through an H.F. amplifier to a mixer, to produce an I.F. signal which is applied to an FM demodulator 7. A synthesizer circuit 5 such as a PLL type of frequency synthesizer, produces a frequency control voltage which is applied to a local oscillator 4 to control the local oscillator frequency which is applied to mixer 3. The I.F. signal is supplied to each of two frequency dividers 9 and 10, which have respective frequency division ratios N.sub.H and N.sub.L. The resultant output signals from frequency dividers 9 and 10 are applied to inputs of two frequency comparators 12 and 13 respectively, while a reference frequency produced from a reference signal oscillator 11 at a frequency F.sub.S is applied to each of the other inputs of the frequency comparators 12 and 13. Each of the frequency comparators 12 and 13 can be of digital type, for example having the configuration shown in FIG. 5. Such a frequency comparator displays a relationship between phase (frequency) and the DC component of the output therefrom having the form shown in FIG. 4. The outputs from frequency comparators 12 and 13 are transferred through respective low-pass filters (LPFs) 14 and 15, to remove AC components of the comparator output signals and thereby produce respective frequency comparison signals.
For each of the frequency comparators 11, 12 in FIG. 3, if the center frequency of the frequency-divided I.F. signal supplied to the frequency comparator is higher than the reference frequency frequency, then the corresponding frequency comparison signal goes to a high logic level (hereinafter referred to as the H level), while if the center frequency of the frequency-divided I.F. signal is lower than the reference frequency frequency, then the corresponding frequency comparison signal will go to a low logic level (hereinafter referred to as the L level). FIGS. 6A and 6B are graphs showing the variation of of the frequency comparison signals produced from LPFs 14 and 15 respectively, in which I.F. signal center frequency are plotted along the horizontal axis and the level of the frequency comparison signal along the vertical axis. Designating the specified center frequency of the I.F. signal as f.sub.O, the reference frequency frequency and the frequency division ratios N.sub.H, N.sub.L must satisfy the following relationship EQU F.sub.S .times.N.sub.L &lt;f.sub.O &lt;F.sub.S .times.N.sub.H
As can be understood from FIG. 6, when the first frequency comparison signal (i.e. the output from LPF 14) is at the H level, the center frequency of the I.F. signal will be higher than the specified frequency by an amount which exceeds a predetermined limit frequency (F.sub.S .times.N.sub.H). Thus it is necessary to apply fine adjustment to the control voltage of local oscillator 4, by means of synthesizer circuit 5, such as to correct this frequency error. Similarly if the second frequency comparison signal (i.e the output from LPF 15) is at the L level then this indicates that the center frequency of the I.F. signal frequency is lower than the specified I.F. frequency by a specific amount, i.e. is lower than a lower limit frequency (F.sub.S .times.N.sub.L). The synthesizer circuit 5 must therefore apply correction to the local oscillator frequency in a direction to correct this frequency error.
It is necessary for an FM receiver for reception of earth satellite transmissions to have an A.F.C. control accuracy which is within at least .+-.300 kHz, i.e. the frequency error range should .+-.300 kHz with respect to the specified center frequency of the I.F. signal. Thus each of the quantities (F.sub.S .times.N.sub.H -f.sub.O), and (f.sub.O -F.sub.S .times.N.sub.L) should be selected to be within 300 kHz.
For example if the specified center frequency of the I.F. signal f.sub.O is 510 MHz, the reference frequency frequency F.sub.S is 10 kHz, one of the frequency division ratios N.sub.H is 51030 and the other frequency division ratio N.sub.L is 50970, then the above relationships are satisfied.
With the system of FIG. 3 described above, the I.F. signal is frequency divided by two frequency dividers having mutually different frequency division ratios, and the frequency-divided output signals are compared in frequency with a reference frequency, whereby a frequency error which causes the center frequency of the I.F. signal to depart from a predetermined frequency error range (centered on the specified if frequency) can be accurately detected. The frequency stability of this system is determined by the reference frequency, so that if a highly stable and accurate oscillator circuit such as a quartz crystal controlled oscillator is utilized to produce the reference frequency, an A.F.C. system can be realized which has a high degree of frequency control accuracy.
However with an A.F.C. system of the form shown in FIG. 3, although the frequency accuracy is extremely high and the long-term frequency drift is almost zero, it is necessary to employ two separate frequency dividers, two frequency comparators, and two low-pass filters. Thus this system has the disadvantage that the manufacturing cost will be relatively high.